The present invention is directed generally to pacemakers, and more particularly to a battery consumption monitoring circuit for use in implantable pacemakers.
Implantable electronic medical devices such as cardiac pacemakers are typically powered by a battery which is implanted with the pacemaker as a single unit. Once the pacemaker has been implanted, the battery is unaccessible and there is no convenient method by which to test its state of depletion. However, because the life of the patient often depends on proper operation of the pacemaker, which in turn is dependent on the condition of the battery, it is imperative that some means of accurately predicting the end of battery life be available to the patient or his physician.
Previous attempts at predicting battery life have centered on periodically subtracting the estimated consumption from the theoretical amp-hour capacity of the battery, and scheduling replacement of the battery as a theoretical limit is reached. This method has not been entirely satisfactory, since the operating parameters of the implanted pacer, and hence the power consumed by the pacer, may vary widely over the life of the pacer, either because of physiological changes in the patient, or because of marked changes in the patient's physical activity. In the past it has been necessary to allow an undesirably wide margin of error in the battery life prediction to guard against these eventualities, thereby forcing premature surgical replacement of the implanted pacer and its battery in many cases, with the attendant risks of complications to the patient.
Other attempts at predicting battery life have involved taking instantaneous current drain measurements by telemetry and from these statistically predicting battery life. However, these measurements do not necessarily remain constant once the patient has left the physician's office, and consequently undesirably large safety factors must still be applied if premature depletion of the battery is to be avoided in all cases.
Various systems are in use for indicating to a patient that the battery of an implanted pacer is nearing depletion, as by varying the output frequency of the pacer, or by providing a marker pulse readable on an ECG. However, such signaling arrangements are only available after-the-fact of impending battery depletion, and may in practice not be immediately noticed by the patient. Furthermore, such warning systems do not give an indication to the user of actual battery life remaining from which he can plan for battery replacement at a convenient time.
The present invention overcomes these problems by providing a continuous indication of the actual quantity of charge consumed from the battery. Since the quantity of charge available from a given battery can be accurately calculated, the remaining battery life can be readily determined with accuracy.